Particularly among girls, the adolescent transition presents a unique vulnerability period for the development of eating risk behaviors that ultimately become very strong predictors of later eating pathology. Recent research has suggested that eating risk behaviors may be promoted by peer influence processes. However, this hypothesis has not been rigorously tested. This proposal will extend previous peer influence research by using an experimental design, examining theoretically-derived hypotheses from social psychology and developmental psychopathology literature, referencing specific peers as potential modes of influence, and examining eating risk behaviors as outcomes. Thus, this study will offer an opportunity for 'translational research'to understand peer influence of eating risk behavior, and identify targets for prevention efforts. This study also will utilize an innovative, technology-based experimental paradigm. A sample of 350 9th and 10th grade adolescent girls (aged 13-16;25% minority) will complete the study;a subset of 120 participants also will complete an established experimental paradigm to examine conformity to social norms of eating risk behavior in a simulated 'chat-room.'The experimental manipulation will involve girls'exposure to social norms communicated by either: 1) thin and sociometrically (i.e., peer nominated) popular peers;2) thin and sociometrically average peers;or 3) heavy and sociometrically average peers. Outcome variables will include adolescents'reports of body dissatisfaction, internalization of the thinness norm, and eating pathology following exposure to peers'norms. Body Mass Index percentile based on age and sex (BMI-by- age) will be examined as a moderator of adolescent girls'socialization of eating risk behavior. It is hypothesized that: 1) adolescents exposed to eating risk behavior norms communicated by thin and sociometrically popular peers will be more likely to conform to these behaviors than participants in the two other conditions, and 2) higher levels of girls'BMI-by-age will be associated with higher levels of conformity to eating risk attitudes, regardless of experimental condition. Structural equation modeling will be used to examine between-subjects factors, as well as BMI-by-age as a moderator of the association between the between-subjects factors and conformity to eating risk attitudes. Findings from this study will elucidate how peer influence processes may be important in the socialization of eating risk behaviors, and what factors may be altered within the peer context to mitigate the effects of peer influence. PUBLIC HEALTH RELEVANCE: This study will help to identify why some adolescent girls are more influenced by peers than others, and how peer influence processes are related to the development of eating pathology risk factors.